With advancing miniaturization in semiconductor technology, so-called resolution enhancement techniques (RET) are increasingly being employed. These techniques involve improving the resolution capability of an optical imaging system by utilizing or influencing the wave and phase properties of the imaging light beam beyond that of a straightforward projection of the pattern. Examples of such techniques are the use of oblique light illumination in the illumination optical system of an exposure apparatus, so-called off-axis illumination, or phase shift masks, which are differentiated further according to the type of alternating or attenuated phase shift masks, etc.
Specifically, these techniques are adapted to the requirements of a pattern that is currently to be transferred onto a semiconductor wafer. Problems occur, however, when partial patterns that are subject to different requirements made of the RET exist within a pattern. Since the imaging of a mask can be performed only under uniform conditions, the common process window existing for the projection is thereby degraded. It is necessary to make compromises between the desired resolution, the contrast, the positional accuracy, the permissible depth of field range, etc. The problem can be explained in more detail using an example. The lithographically formed structure elements of active zones of a memory component using trench capacitor technology are close to the resolution limit of an imaging system for memory cells. The peripheral structures of the memory component likewise comprise active zones, but are not subject to the high requirements of structure density and width. For example, these structures may have a width that is a factor of three to four times greater than that of active zones in the cell region.
One conventional solution approach involves performing a double exposure. In this case, the resolution-critical line-space grating structures of the cell region are transferred by a first oblique illumination, e.g., a dipole illumination. As an alternative to this, it is also possible to use an alternating phase shift mask for the projection onto the semiconductor substrate, the appropriate line-space grating structures being formed on the mask.
By contrast, those spaces and lines which represent structure elements from the periphery are usually imaged by a three-tone mask under annular illumination. Chromium masks are also often used instead. For this purpose, the corresponding pattern portions, while still in the design stage, are taken from the layout of the first mask and combined in a new, second layout, from which is created the second mask for the double exposure.
However, the double exposure also entails considerable disadvantages. First, the time expenditure is doubled owing to the masks that must be changed during each exposure process. This is accompanied by a reduction in productivity. If it is taken into consideration that the greatest proportion of costs in semiconductor production arises as a result of the apparatus time in the area of photolithography, then this disadvantage also directly influences the cost budget. The production costs for the mask are also doubled.
Second, an alignment must be carried out in each case for the exposure of the relevant lithographic plane (in the example: active zones). Unavoidable errors as a result of a limited alignment accuracy additionally restrict the predetermined tolerance budget.
In the example of forming active zones in the cell and peripheral regions of a memory component, more extensive problems arise in connection with the imaging of the so-called MUX space, which serves for forming multiplexers in the integrated circuit. The MUX space lies in the peripheral region in direct proximity to the edge of the cell region and comprises a complex, semi-laterally closed line-space structure having a line-to-space ratio of approximately 1.5. If the corresponding space structure is formed as an alternating phase shift mask, for instance, then phase conflicts inevitably arise at the branchings of the spaces.
Therefore, one goal is to transfer the layout into the image plane or onto the substrate in the context of a single exposure. Solution approaches are known for this purpose, too. By way of example, it has been attempted, by using a symmetrical quadrupole illumination adapted to the layout of the mask (e.g., chromium or attenuated phase shift mask), to simultaneously transfer both the line-space grating and the structure elements arranged peripherally with respect to the cell region, in particular also the MUX space, in just one exposure.
The type of illumination originating from the radiation source thus has a significant influence on what orders of diffraction contribute in what way to the image construction in the image plane. In this case, the radiation source is to be understood to be an “effective source” in which not only the finite extent of the light-generating source itself plays a part, rather the form of the illumination pupil arranged at a position between light-generating source and mask in the beam path of the exposure apparatus is also of importance.
By setting the illumination pupil, it is possible to realize oblique light, dipole, quadrupole, annular, rectangular, or circular illumination. The pupil is situated in a Fourier-transformed plane relative to the mask or in a conjugate plane relative to the radiation source.
A quadrupole illumination is applied in the prior art by openings that are essentially shaped in square fashion, or generally light-transmissive bright zones, having an identical size being positioned symmetrically and equidistantly from the optical axis defined by the beam path on axes oriented perpendicular to one another in the pupil plane. In the example of the line-space grating, two of the square pupil bright zones are thereby respectively oriented parallel to the orientation of the grating, and the other two bright zones are oriented perpendicular to the orientation as soon as the mask is introduced in the beam path at the location of the mask plane.
One disadvantage of this type of illumination exists insofar as the two bright zones arranged along an axis parallel to the grating orientation can scarcely contribute to the image construction of the lines and spaces of the grating in the image plane. Therefore, they degrade the aerial image contrast considerably, with the result that the size of the process window is significantly reduced. A process window is defined by an interval of permissible combinations of focus and dose values for an exposure.
A further negative consequence is that the value of the MEEF (mask error enhancement factor) for the line-space grating is drastically increased, whereby the uniformity with which desired line widths are obtained is impaired. The MEEF represents the nonlinear behavior during the transfer of errors present anyway on the mask (structure width) onto the semiconductor wafer, which generally commences in the case of structure elements having widths close to the resolution limit.
One alternative involves the use of a hybrid mask which combines elements of an alternating phase shift mask for the formation of the line-space grating and also elements of a chromium mask for the peripheral structures with one another. The application of the technology of alternating phase shift masks makes it possible to achieve a high contrast value in the aerial image arising in the image plane, a large process window, and a low MEEF value for the line-space grating.
However, the quality of the imaging of the line-space grating is coupled to a considerable extent to the type of illumination of the lines and spaces formed in alternating phase shift mask technology. Line-space gratings formed in this technology typically produce particularly high contrast values and low MEEF values during the projection precisely when a virtually coherent illumination is used. An illumination that is as coherent as possible is obtained by an arrangement of the bright zones in the pupil plane that lies close to the optical axis—also called zero point or origin.
Known embodiments of pupils that have been used especially in combination with hybrid masks therefore form centrally centered, virtually coherent effective sources. Such a pupil selection conflicts with the imaging of the peripheral structures, which is preferably to be performed using oblique light illumination (dipole or annular). The result is a significantly reduced imaging quality of the peripheral structures. This applies particularly to the MUX space as well, in particular when the depth of field range is intended to be utilized (i.e., high defocus values are set). If, on the other hand, an oblique light illumination adapted to the peripheral structures is set in the case of the hybrid mask, then a contrast value of less than 0.45 is obtained for structure elements of the line-space grating in the aerial image that arises in the image plane. Such a low contrast value is no longer acceptable for a subsequent processing of a resist on a wafer that is exposed in the image plane.